Abstract : Reproduction of the St. Louis encephalitis (SLE) virus is demonstrated to occur in primary and continuous tissue cultures of various origin. The virus is the most actively reproduced in cultures of chicken fibroblast cells (CFC), human embryo, fibroblasts (HEF), and VNK-21. The cytopathic effect of the SLE virus is manifested only in the CFC. The formation of an interferon on contamination with the SLE virus is observed in cultures of CFC and L cells. The phenomenon of interference of the SLE virus with the Venezuelan equine encephalomyelitis (VEE) virus is observed in the same tissue cultures. It has been found that the interference method may be used for titration of the SLE virus in tissue cultures. The degree of interference in the infected cultures depends on the amount of interferon produced by them. (Author)